Support non-free GitHub Copilot Chat endpoints

[isometry]

Check for existing issues

• Completed

Describe the feature

Zed currently hard-codes the non-business Copilot Chat API endpoint as https://api.githubcopilot.com/chat/completions (ref. https://github.com/zed-industries/zed/blob/main/crates/copilot/src/copilot_chat.rs#L16).

In corporate environments in which access to non-Business Copilot tiers has been blocked, GitHub's guidance is to block access to api.githubcopilot.com, api.individual.githubcopilot.com and copilot-proxy.githubusercontent.com; leaving access only to api.business.githubcopilot.com.

To support the correct endpoints for both Individual and Business Copilot tiers, the existing COPILOT_CHAT_COMPLETION_URL variable should be updated to support override via settings.json.

Zed Version and System Specs

Zed: v0.168.2 (Zed)
OS: macOS 15.2.0
Memory: 64 GiB
Architecture: aarch64

If applicable, add mockups / screenshots to help present your vision of the feature